First Report of Pectobacterium carotovorum subsp. brasiliense Causing Blackleg and Stem Rot Disease of Potato in Russia

Abstract

Blackleg and stem rot of potato (Solanum tuberosum) were observed in several commercial fields in the central part of Russia with up to 40% incidence during 2012 to 2017. Symptoms included decay of internal root and stem tissues, blackening of petiole vascular bundles, and partial yellowing of leaves. The disease can cause serious potato yield losses in the field and storage in Russia. Pectobacterium carotovorum subsp. carotovorum, P. atrosepticum, Dickeya dianthicola, and D. solani are the main causal agents of blackleg disease in Russia (Kornev et al. 2012). Surveys conducted in commercial fields produced multiple samples showing stem rot symptoms. Bacteria were isolated from symptomatic tissues by suspending 1-cm stem sections with lesions in sterile tap water for 10 min, followed by serial dilutions on crystal violet pectate agar (CVP). Bacterial colonies producing pitting on CVP were reisolated and purified on nutrient broth yeast extract medium. Characterization of three isolates provided evidence that a previously unreported blackleg/stem rot pathogen was present in the surveyed region. F126 (from potato ‘Lady Clare’, Samara Region), F128 (from potato ‘Sante’ from Kaluga Region), and F140 (from Moscow region) were gram negative, facultatively anaerobic, and able to grow at 37°C on nutrient agar. They produced typical stem rot symptoms in potato plants inoculated with 10⁷ CFU/ml of the pathogen by stem injection and incubated at 25°C for 48 h. The strains were reisolated successfully from symptomatic potato to complete Koch’s postulates. These strains were negative in polymerase chain reaction (PCR) analyses using specific primers Y45/Y46 for the detection of P. atrosepticum and ADE1/ADE2 for Dickeya (Czajkowski 2011). In contrast, all strains yielded an amplicon (322 bp) in a PCR with primers Br1f and L1r for P. carotovorum subsp. brasiliense (Duarte et al. 2004). 16S rRNA was amplified using universal primer set F27/R1492 (Monciardini et al. 2002). The strains F126, F128, and F140 (GenBank accessions MG570140, MG570141, and MG570142) had 99% sequence identify with 16S rRNA of P. carotovorum subsp. brasiliense type strain BC1 (CP009769.1). Additional loci representing two housekeeping genes were amplified and sequenced: gyrB (MG570137, MG570138, and MG570139) and recA (MG570134, MG570135, MG570136). Primer sets used were as reported by Young and Park (2007) and Dauga (2002). All strains showed 97 to 98% similarity with type strain P. carotovorum subsp. brasiliense BC1. To our knowledge, this is the first report of blackleg and stem rot caused by P. carotovorum subsp. brasiliense on potato in the Russian Federation. According to the data of several diagnostic laboratories, P. carotovorum subsp. brasiliense occurred in >20% potato seed stocks harvested in 2017 and was the predominant blackleg pathogen in fields in the Moscow region in 2018.